So Happy Days does not count except for Joanie & Chachi. Andy Griffith Show does count and it is 1 for 1 on success.
Family Ties, only Mallory would count, Michael J. Fox was 21 when Family Ties started.
On Partridge Family; David Cassidy was 20, Susan Dey was 18 so neither count.
The 70s Show only Milas Kunis was under 18, so they’re 1 for 1.
If Freaks and Geeks counts as a sitcom, it probably wins. The 3 younger kids were under 18 and all successful and still working as actors. John Francis Daley, Samm Levine & Martin Starr. The older kids have done great overall, but don’t count, even Seth Rogan was 19.
So what sitcom child actors where successful as non-child actors as a high percentage.
Captain Video and his Video Rangers. Not a sitcom, but Don Hastings was 14 when it started and then was on TV soap operas (notably As the World Turns) as a regular for over 50 years.
Neil Patrick Harris was 16 when Doogie Howser started on the air.
Mackenzie Phillips, who had appeared in American Graffiti a few years earlier, was sixteen when One Day at a Time made its debut in 1975. Valerie Bertinelli, who was born in 1960, had only one screen credit (an appearance on Apple’s Way) before being cast as Barbara Cooper.
You didn’t didn’t specify ‘success as an actor’ though. However it is something of a borderline case I think as their financial success outside of acting started when the show was still on.
A pretty good example, even if the title actor is now radioactive.
Lisa Bonet, Malcolm Jamal-Warner, Tempestt Bledsoe, Keshia Knight Pulliam, and Raven-Symone were all minors when they first appeared on the show (Bonet, the oldest, was 16 when the show premiered), and all have had reasonably successful careers as adult actors.
Fred Savage - still acting
Danica McKellar - several dozen Hallmark Xmas movies
Jason Hervey - still acting
Probably others, it’s been awhile since I’ve seen it